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    It is fully 100mm thick, over underfloor, placed 19 days ago.
    I was told it could be tiled over in 2 weeks,
    Still getting readings of 18% moisture,(& got 24% last week on the 2 week deadline) as against a bit of 20 year old scratch plastered wall(internal) giving me 12% on the same (timber) moisture meter.
    I got the Larsen brand Acrylic primer and Larsen "flexible rapid set" adhesive ready to go.
    I suspect good to go, but dont want to take any chances at this stage.
    • CommentAuthoradi
    • CommentTimeAug 22nd 2017
    Normally you don't have anhydrate sceeds that thick which Is why it drys in less time than a concrete screed as its normally thinner.( actually drys at the same rate as concrete)

    Will dry at 1mm per day till for first 40mm and 0.5mm per day after that. That means it should take 160 days!


    If you have under floor heating you could speed up the process by turning it on to force dry the screed.
    Thanks adi, my otherwise genuinely "all knowing" builder was apparently misinformed, but better that I asked!
    I will certainly consider tickling up the underfloor, to speed things up.
    thanks again
    Ho Hum?
    The guy at the supplying plant was absoutely adamant that 2 weeks for 100mm was plenty, before tiling, over underfloor.
    They had had it tested, as in a sample from within the 100mm thickness laboratory tested.
    Anyway I was "hoking" a 100mm deep hole to move the air supply for the stove a bit sideways, and it certainly seems to be as dry right throughout the full depth.
    I think I will proceed.
    thanks anyway
    • CommentAuthorgyrogear
    • CommentTimeAug 22nd 2017
    <blockquote><cite>Posted By: orangemannot</cite>it certainly seems to be as dry right throughout the full depth.</blockquote>

    It might appear so, but that is probably because the "wall" (surface) of the hole is dry: the heart of the pour might be a different matter...

    Was "hoking" down past the side of a 110mm Wavin, out a good 100mm forby.
    I do understand what you are saying mind.
    • CommentAuthorCWatters
    • CommentTimeAug 22nd 2017
    Needs to be dry. Helps to remove laitance..


    "Before laying onto Calcium Sulfate screeds it is vital to ensure that the moisture content is at the correct level for the required floor finish. Typical values for moisture sensitive floor finishes are less than 0.5% Water by Weight or less than 75% relative humidity for impermeable floors and less than 1% w/w for more permeable floors."

    "Before laying tiles or other finishes onto Calcium Sulfate screed one of the most important tasks is the removal of laitance.

    Laitance is a weak layer of fine particles deposited on the surface of the screed as the anhydrite cures. This layer is too weak to tile onto and can also inhibit drying of the screed. Many of the failures we see are as a result of contamination of the adhesive by laitance – highlighting the importance of carrying out this step."
    Laitance=dust, yes?
    I was told this screed will not be dusty, nor is it, it has a really good tight sound surface, I suppose I should ask precisley what specification it is/was, as opposed to the: "expensive super-duper stuff".

    But the supplying firm specialize in floor screeds are are highly regarded within the trade here in NI.

    Anyway 0.5% W by W or indeed 75% relative humidity means nowt to me, as (i) I have no way of checking the 0.5% fig., nor (i) can I understand the 75% relative humidity(which I thought was only used for atmospheric moisture?)

    But I do understand my £50.00 timber moisture meter's read-out figures, i.e. 10%, 12%, 20% moisture etc etc.
    Especially if used to compare against other similar, & known to be completely dry, masonry surfaces.
    I am aware that the surface is not reflective of the core,
    but when I left the large thick porclain tiles down overnight, last night,( looking at various possible tile patterns),
    I noted, that whilst the underside of the tile was absolutly not sweaty the next morning, this area of floor gave higher surface moisture readings, in the low 20's, than the uncovered remainder.
    When I last tried this at 2 weeks, the underside of the tiles was dripping moisture in the morning.
    As I mentioned I am using a same brand acrylic primer, that is the same brand as the tile adhesive.
    edited comment, per a remark below.
    I also noticed the colour difference, between the areas that had been covered overnight, and those that had not, damper=darker.
    Friends of ours were having a resin floor over anhydrite/UFH. It took a long time to dry - many weeks longer than it was supposed to.
    • CommentAuthorMarkyP
    • CommentTimeAug 22nd 2017
    I have 65 - 70mm of anhydrite screed in my extension. It's nearly two months since it was placed and it still isn't fully dry. These screeds take a good while to dry, for 100mm you are looking at 2+ months unless you force dry it with the UFH. If you do use the UFH do it very slowly, start at 25C water temp and then increase a few degrees every couple of days until up to full operating temp. I'm giving my UFH a run this week just to be sure the screed is fully dry.

    laitence - it's not really dusty, more a skin that forms, at least was with my screed. You can scrape/abrade it off in the days after the screed initially sets but then it seems to bake on with time and is very hard to remove. Flooring contractors experienced with anhydrite will often sand the whole floor prior to tiling to ensure laitence removed. I'm having microcrete and the contractor is going to use a 60 grit floor sander to ensure the laitence is completely gone, followed by two part primer with sand blinding for key.

    You can find lots of horror stories on tiling forums about anhydrite either tiled over when still damp or laitence coming loose and tiles blowing. Also take care with adhesive and primer choices, not all get on with anydrite.

    proceed carefully and dont rush it.
    • CommentAuthoradi
    • CommentTimeAug 22nd 2017
    A easy way to tell if you still have moisture is to place a piece of plastic sheet about 2ft * 3ft on the floor with a similar sized piece of ply on top and leave for at least 1 day.

    After 1 day remove plastic sheet and instantly compare colour, if area under plastic sheet is a different colour (darker) to the rest of the floor it's still,drying out.

    A section of my anhydrate screed that was nearly 100mm thick took about 12 weeks to dry. Luckily I wasn't in a rush to tile the floor.
    • CommentAuthorcjard
    • CommentTimeAug 23rd 2017
    Mapei say, if using their mapeker adhesive, to use eco prim-t first. It looks, tastes and smells like SBR, but is way more costly. I didn't take the chance. I also left my screed to dry for about 9 months, then the day before it was due to be tiled the roof leaked in a major way and I ended up with the best part of a thousand litres of water sloshing around inside. You could literally see the screed sucking it up. I plumbed in the UFH and ran it at 32 degrees for 2 weeks, did the "lay a sheet of glass on the screed" test - no condensation on the glass, no discoloration (it's more orangey/wetter looking) of the screed, so i tiled it..

    Force drying it with the UFH definitely helped. Being dry for 9 months before The Deluge probably helped
    • CommentAuthorBeau
    • CommentTimeAug 23rd 2017 edited
    Sorry cant help with drying times as ours had ages due to my rapid diy skills haha. for the tiling I was advised to grind back the surface and apply a special primer if using cement based tile adhesive. The primer was from these guys but cant remember which one off the top of my head http://www.uzin.co.uk/products/ The grinding was easy to do but astoundingly messy. Just used one of these on a little angle grinder https://www.toolstation.com/shop/p62515?searchstr=diamond%20grin
    • CommentAuthorCWatters
    • CommentTimeAug 26th 2017
    Posted By: orangemannotLaitance=dust, yes?

    It's not just dust. It's a weak layer that forms on the surface. Can look like a milky white film or patch where there was obviously more water.
    • CommentAuthorMarkyP
    • CommentTimeAug 29th 2017
    I'd spilt a dollop of SBR on my screed a week or two back and yesterday I removed it in readiness for the flooring contractor. Lifting the edge with a scraper it was then easy to peel the SBR away from the screed and I noticed that the surface skin of the screed came away with it. This mopped up any doubts I had about the importance of surface prep for these screeds. The surface of the screed isn't obivously weak, it was only when this SBR lifted it with ease that it become clear how weakly bonded the laitence is to the main body of the screed.

    my flooring contractor started today, bit of a false start as had the wrong primer for anhydrite, but has already sanded the whole screed with a 60 grit disc on a floor sanding machine.
    • CommentAuthorborpin
    • CommentTimeSep 1st 2017
    Posted By: orangemannotI was told this screed will not be dusty, nor is it, it has a really good tight sound surface, I suppose I should ask precisley what specification it is/was, as opposed to the: "expensive super-duper stuff".

    But the supplying firm specialize in floor screeds are are highly regarded within the trade here in NI.

    I ask if you could go and see one of their other jobs. There are various screeds out there and it may be this is a specific brand that has different properties.

    Ask them if they will guarantee their advice in writing!
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